Severe Weather Possibilities

The atmosphere is ripe and ready to let some pretty strong storms get going on Wednesday & Thursday of this week.

The high levels of humidity have been tough here the last couple of days. This will again be the case coming up on Wednesday & Thursday. That means there is plenty of moisture in the air. This is one of the key ingredients in storm development. There are three and here they are...

There are three ingredients that must be present for a thunderstorm to occur. They are moisture, instability and lift.

As a general rule, the surface dewpoint needs to be 55 degrees of greater for a surface based thunderstorm to occur. A dewpoint of less than this is unfavorable for thunderstorms. Dewpoints at the surface can be less than 55 degrees in the case of "elevated thunderstorms" (I'll explain later)

Instability also decreases as low-level moisture decreases. Instability occurs when air is warmer than the environmental air and rises on its own due to positive buoyancy. Instability is often expressed using positive CAPE or negative LI values. Instability is what allows air in the low levels of the atmosphere to rise into the upper levels of the atmosphere. Without instability, the atmosphere will not support deep convection and thunderstorms. Instability can be increased through daytime heating. Lift is what gives a parcel of air the impetus to rise from the low levels of the atmosphere to the elevation where positive buoyancy is realized. Very often, instability will exist in the middle and upper levels of the troposphere but not in the lower troposphere. Low level stability is often referred to as negative CAPE, convective inhibition, or the cap.

It is lift that allows air in the low levels of the atmosphere to overcome low level convective inhibition. Lift is often referred to as a "trigger mechanism". There are many lift mechanisms. A list of many of them follows: fronts, low level convergence, low level WAA, low level moisture advection, mesoscale convergence boundaries such as outflow and sea breeze boundaries, orographic upslope, frictional convergence, vorticity, and the jet streak. All these processes force the air to rise. The region that has the greatest combination of these lift mechanisms is often the location that storms first develop. Moisture and instability must also be considered. A thunderstorm will form first and develop toward the region that has the best combination of: high PBL moisture, low convective inhibition, CAPE and lifting mechanisms.

The difference between a thunderstorm and a severe thunderstorm is the wind field. For a severe thunderstorm, the ingredients that must be present are moisture, instability, lift and strong speed and directional storm relative wind shear. Ideally, wind will have a veering directional change of 60 degrees or more from the surface to 700 millibars, upper level winds will be greater than 70 mph, and the 850 to 700 mb winds (low level jet) will be 25 mph or greater. Wind shear aids in the following: Tilting a storm (displacing updraft from downdraft), allows the updraft to sustain itself for a longer period of time, allows the development of a mesocyclone, and allows rotating air to be ingested into the updraft (tornadogenesis).

Severe storms also tend to have these characteristics over ordinary thunderstorms: higher CAPE, drier air in the middle levels of the atmosphere (convective instability), better moisture convergence, baroclinic atmosphere, and more powerful lift.

The set-up is here and will be here for a couple of days to see some of those severe storms in our region.

This graphic will update automatically, but I will give you the outlook for Weds & Thurs

The Wednesday severe weather outlook.

So what I am expecting on either of these days will include the possibility of some strong winds in excess of 60 mph or some pretty large hail. There have been several storms rotating and hinting that there could be tornadic development in states to our west. But will there be any of that here? The liklihood is low, but I wouldn't rule it out completely!

Updated: 03/02/2015
- People who live near flood-prone creeks and streams will be keeping a close eye on the water on Tuesday. Folks who live near Stoner Creek in Bourbon County tell us they're already taking precautions.

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